In conventional medical diagnostic imaging the object is to obtain an image of a patient's internal anatomy with as little X-radiation exposure as possible. The fastest imaging speeds are realized by mounting a dual-coated radiographic element between a pair of fluorescent intensifying screens for imagewise exposure. About 5 percent or less of the exposing X-radiation passing through the patient is absorbed directly by the latent image forming silver halide emulsion layers within the dual-coated radiographic element. Most of the X-radiation that participates in image formation is absorbed by phosphor particles within the fluorescent screens. This stimulates light emission that is more readily absorbed by the silver halide emulsion layers of the radiographic element. Crossover of light from one fluorescent screen to an emulsion layer on the opposite side of the support of the radiographic element results in a significant loss of image sharpness. For medical diagnostic imaging, film contrast typically ranges from about 1.8 to 3.2, depending upon the diagnostic application. Crossover is minimized. In the highest speed diagnostic dual-coated radiographic elements, those employing spectrally sensitized tabular grain emulsions, crossover typically can range up to about 25% in the absence of other crossover control measures. In fact, it is common practice to add processing solution decolorizable dye particles to reduce crossover to near zero. X-radiation exposure energies vary from about 25 kVp for mammography to about 140 kVp for chest X-rays.
Following imagewise exposure diagnostic radiographic elements receive aqueous processing in a rapid-access processor to produce a dry, viewable silver image in 90 seconds or less. For example, the Kodak X-OMAT M6A-N.TM. rapid access processor employs the following processing cycle:
______________________________________ Development 24 seconds at 35.degree. C. Fixing 20 seconds at 35.degree. C. Washing 20 seconds at 35.degree. C. Drying 20 seconds at 65.degree. C. ______________________________________
with up to 6 seconds being taken up in film transport between processing steps.
A typical developer exhibits the following composition:
______________________________________ Hydroquinone 30 g Phenidone .TM. 1.5 g KOH 21 g NaHCO.sub.3 7.5 g K.sub.2 SO.sub.3 44.2 g Na.sub.2 S.sub.2 O.sub.3 12.6 g NaBr 35.0 g 5-Methylbenzotriazole 0.06 g Glutaraldehyde 4.9 g Water to 1 liter/pH 10.0 ______________________________________
A typical fixer exhibits the following composition:
______________________________________ Sodium thiosulfate, 60% 260.0 g Sodium bisulfite 180.0 g Boric acid 25.0 g Acetic acid 10.0 g Water to 1 liter/pH 3.9-4.5 ______________________________________
Dual coated radiographic elements intended for rapid access aqueous processing typically employ high bromide {111} tabular grain emulsions that contain less than 3 mole percent iodide, based on silver. Limiting iodide to less than 3 mole percent facilitates rapid access aqueous processing.
Examples of radiographic element construction for medical diagnostic purposes as well as exposure and processing are provided by Abbott et al U.S. Pat. Nos. 4,425,425 and 4,425,426, Dickerson U.S. Pat. No. 4,414,310, Kelly et al U.S. Pat. Nos. 4,803,150 and 4,900,652, Tsaur et al U.S. Pat. No. 5,252,442, and Research Disclosure, Vol. 184, August 1979, Item 18431.
Photothermographic imaging systems have been employed for producing silver images. Typically these imaging systems have exhibited very low levels of radiation-sensitivity and have been utilized primarily where only low imaging speeds are required. The most common use of photothermographic elements is for copying documents. Summaries of photothermographic imaging systems are published in Research Disclosure, Vol. 170, June 1978, Item 17029, and Vol. 299, March 1989, Item 29963.
Reeves U.S. Pat. No. 4,435,499, which was the first to teach the use of tabular grain emulsions in photothermographic elements, identified a clear preference for tabular grain emulsions in which tabular grains account for at least 70 percent of total grain projected area, have an average ECD in the range of from 0.30 to 0.45 .mu.m, and have an average aspect ratio of from 5 to 15. Notice that the maximum preferred average ECD of the tabular grains of Reeves is well below the typical minimum ECD of 0.6 .mu.m of tabular grain emulsions typically present in radiographic elements. The lower ECD's of Reeves also resulted in a maximum preferred aspect ratio of 15, which is below the typical average aspect ratio of tabular grain emulsions contained in radiographic elements. Thus, the preferred emulsion selections of conventional radiography and Reeves for photothermography, respectively, are at least divergent, if not mutually exclusive.
The following patents relating to photothermography illustrate that tabular grain emulsions have from time-to-time been included among possible alternative silver halide emulsions:
Frank et al EPO 0 654 703 A1(note page 7, line 39); PA1 Clark et al U.S. Pat. No. 4,504,568 (note column 4, line 51); and PA1 Bailey et al U.S. Pat. No. 5,468,587 (note column 15, lines 46-58). PA1 (1) a vehicle (i.e., binder and peptizer), PA1 (2) radiation-sensitive silver halide grains, PA1 (3) a light-insensitive source of silver, and PA1 (4) a reducing agent for the light-insensitive source of silver. PA1 (a) having {100} major faces, PA1 (b) containing greater than 70 mole percent chloride, based on silver, PA1 (c) exhibiting an average thickness of less than 0.3 .mu.m, and PA1 (d) exhibiting an average equivalent circular diameter of greater than 0.6 .mu.m. PA1 ECD/t&gt;5 PA1 ECD/t.sup.2 &gt;25 PA1 ECD is the effective circular diameter of the tabular grains in micrometers (.mu.m) and PA1 t is the thickness of the tabular grains in .mu.m. In arriving at the average aspect ratio or average tabularity for a tabular grain population it is contemplated to average separately the ECD's and the thicknesses of the tabular grain population and then to obtain the quotient required by relationships I and II. PA1 (1) a vehicle, that includes as a minor component the peptizer associated with the grains in their preparation and, as a major component, a binder; PA1 (2) photosensitive silver halide grains, including high chloride {100} tabular grains, as described above; PA1 (3) a light-insensitive silver source; and PA1 (4) a reducing agent for the light-insensitive silver source. PA1 M is at least one of the metals yttrium, lanthanum, gadolinium or lutetium, PA1 M' is at least of the rare earth metals, preferably dysprosium, erbium, europium, holmium, neodymium, praseodymium, samarium, terbium, thulium, or ytterbium, PA1 X is a middle chalcogen (S, Se or Te) or halogen, PA1 n is 0.002 to 0.2, and PA1 w is 1 when X is halogen or 2 when X is chalcogen.
The fact that none of the Examples in Frank et al, Clark et al and Bailey et al employ a tabular grain emulsion provides clear evidence of the established preference for non-tabular grain emulsions in photothermographic systems.
Frank et al, Clark et al and Bailey et al are all directed to dye image transfer systems, which in itself may account for their willingness to consider tabular grain emulsions as a possible alternative. Whereas main-stream photography fixes out undeveloped silver halide grains to impart image stability (thereby increasing image discrimination, D.sub.max -D.sub.min) and to reduce light scatter on viewing. there is no convenient mechanism for removing undeveloped silver halide grains from photothermographic elements. Accordingly, there is a clear trend in photothermography toward image transfer systems, since they allow undeveloped silver halide grains to be hidden from view. Image transfer in itself degrades image sharpness, thereby limiting imaging uses to those that do not require significant magnification. Image transfer also increases the number of layers that must be constructed. Thus, Frank et al, Clark et al and Bailey et al escape some of the major disadvantages of retained image photothermographic systems only by incurring the known limitations of image transfer systems.
High chloride {100} tabular grain emulsions and their use in mainstream photographic systems are illustrated by Maskasky U.S. Pat. Nos. 5,264,337, 5,292,632 and 5,275,930, House et al U.S. Pat. No. 5,320,938, Brust et al U.S. Pat. No. 5,314,798, Szajewski et al U.S. Pat. No. 5,356,764, and Budz et al U.S. Pat. No. 5,395,746.